This invention relates to a method and apparatus for sludge lancing suitable for use in removing the sludge deposits on the tube sheet of a nuclear steam generator. The invention has particular application to removal of residual sludge deposits.
A typical nuclear steam generator comprises a vertically oriented shell and a plurality of inverted U-shaped tubes disposed in the shell so as to form a tube bundle. Each tube has a pair of elongated vertical portions interconnected at the upper end by a curved bight portion, so that the vertical portions of each tube straddle a center lane or passage through the tube bundle. The tubes are dimensioned and arranged so that, on each side of the center lane or passage, the vertical tube portions are disposed in an array of parallel rows separated by lanes and parallel columns separated by channels, with the rows and columns extending perpendicular to each other.
A tube sheet supports the vertical portions of the tubes at their lower ends. The vertical tube portions on one side of the center lane are connected to a primary fluid inlet plenum and the those on the other side of the center lane are connected to a primary fluid outlet plenum. The primary fluid, having been heated by circulation through the reactor core, enters the steam generator through the primary fluid inlet plenum, is transmitted through the tube bundle and out the primary fluid outlet plenum. At the same time, a secondary fluid or feedwater is circulated around the tubes above the tube sheet in heat transfer relationship with the outside of the tubes, so that a portion of the feedwater is converted to steam which is then circulated through standard electrical generating equipment.
Sludge, mainly in the form of iron oxides and copper compounds, along with traces of other metals, settle out of the feedwater onto the tube sheet. The sludge deposits provide a site for concentration of phosphate solution or other corrosive agents at the tube walls that results in tube thinning. Accordingly, the sludge must be periodically removed.
One known method for removal of the sludge is referred to as the sludge lance-suction method. Sludge lancing consists of using high pressure water to break up and slurry the sludge in conjunction with suction and filtration equipment that remove the water-sludge mixture for disposal or recirculation. A lance emits a high-velocity water jet or stream substantially perpendicular to the movement of the lance, i.e., parallel to the columns of tubes.
This method works well as long as the sludge height is about one inch or higher on the tube sheet. This is because the higher sludge levels limit the spread of the lancing flow to the channel being processed, and because the leaving of a residual layer one-half to one inch deep is minor if, e.g., six inches of sludge have been removed. However, if the initial sludge depth is one inch or less, a one-half inch layer represents only fifty percent sludge removal. The removal of this residual sludge is difficult due to the tendency of the sludge lance jet to scatter the dislodged sludge particles laterally into already cleaned areas, instead of sending it down the tube channel to the periphery of the tube sheet where the peripheral flow can transport the sludge to the suction pipes for removal.